[llvm-dev] [RFC] Asynchronous unwind tables attribute

[Fāng-ruì Sòng via llvm-dev]

On Sun, Nov 21, 2021 at 2:11 AM Momchil Velikov
<momchil.velikov at gmail.com> wrote:
>
> On Sun, 21 Nov 2021 at 01:02, Fāng-ruì Sòng <maskray at google.com> wrote:
>>
>> On 2021-11-21, Momchil Velikov wrote:
>> >On Sun, 21 Nov 2021 at 00:33, Fāng-ruì Sòng <maskray at google.com> wrote:
>> >
>> >> On 2021-11-21, Momchil Velikov wrote:
>> >> >         | nounwind 0  |  nounwind 1
>> >> >----------+-------------+--------------
>> >> >uwtable 0 | <full,no>   |  <no,no>
>> >> >----------+-------------+--------------
>> >> >uwtable 1 | <full,no>   |  <full,no>
>> >> >----------+-------------+--------------
>> >> >uwtable 2 | <full,min> | <min, min>
>> >> >----------+-------------+--------------
>> >> >uwtable 3 | <full,full> |  <full,full>
>> >> >
>> >> >where
>> >> > - "full" means full unwind info - CFA, CSRs, return address
>> >> > - "min" is minimal, sans CSRs,
>> >> > - "no" is, well, no unwind info and
>> >> > - "<p,e>" is the kind generated for prologues and epilogues,
>> >> respectively.
>> >>
>> >> uwtable 1/nounwind 1: <full,no>
>> >> uwtable 2/nounwind 1: <min,min>
>> >>
>> >> Why is there a full->min transition for the generated prologue?
>> >>
>> >
>> >Because for a synchronous unwind table it makes only sense for the prologue
>> >to be full, <min, no> is
>> >unusable combination, whereas <full, no> is usable for a debugger (it's
>> >basically what we have now for most backends).
>>
>> I wanted to ask why the prologue information has degraded from full to
>> min when transiting from uwtable 1 to uwtable 2.
>>
>> I do not understand why moving from uwtable 1 to uwtable 2 is not monotonic.
>
>
> It's not that it was degraded in the case for "uwtable=2,nounwind=1", but that it was
> "too much" for "uwtable=1,nounwind=1".  One could generate "<min,no>" there, but that
> serves no purpose - it's unusable for debugging, and for profiling, one would
> be better off with the "<min,min> variant. Also, this is the current state, and
> degrading *that* could be viewed as a regression.

The argument with keeping <full,no> for "uwtable=1,nounwind=1" as the
current state is fine.
But then why is <min,min> for "unwtable=2,nounwind=1" not a problem
for debugging?